The present invention is directed to motor vehicle electrical systems including a storage battery and an engine-driven alternator, and more particularly to a multi-mode converter for coupling the storage battery to the alternator.
In general, a motor vehicle electrical system derives its power from a 12-volt storage battery and an engine-driven alternator, which also supplies charging current to the storage battery during engine operation. In most powertrain configurations, the engine is designed to operate over a wide speed range, and the alternator is subject to a corresponding speed variation. Thus, while the battery voltage is relatively constant under most operating conditions, the alternator produces an output voltage that varies in proportion to its shaft speed. To reconcile the inherent incompatibility, a voltage regulator controls the excitation of an alternator field winding to maintain the alternator output voltage at a desired value, such as 14-volts. Unfortunately, this significantly restricts the alternator""s efficiency and available power output.
Various proposals foe improving the efficiency and power output of an engine driven alternator utilize a converter arrangement to better match the alternator to the load circuit. For example, the U.S. Pat. No. 4,419,618 proposes coupling the alternator to the output rectifier bridge and load circuit through a multiple-tap transformer in which one of a number of possible connections between the alternator output windings and the transformer is selected based on the alternator shaft speed. Another approach, disclosed in the U.S. Pat. Nos. 4,827,393 and 5,483,146, for example, proposes coupling the alternator to the load circuit through a switching DC-to-DC converter, where the converter switching duty-cycle is controlled to regulate the converter output voltage to a desired load voltage. The U.S. Pat. No. 4,827,393 applies to a permanent magnet alternator, whereas the U.S. Pat. No. 5,483,146 applies to a wound field alternator.
While the above-mentioned proposals are capable of improving the efficiency and power output of an engine driven alternator, they also have limited functionality and are relatively expensive to implement due to the required hardware and control electronics. For example, the system described in U.S. Pat. No. 4,419,618 requires an expensive and bulky transformer and numerous bi-directional semiconductor switches. The systems described in U.S. Pat. Nos. 4,827,393 and 5,483,146 require continuous variable duty cycle control and operate only in a buck mode. Accordingly, what is desired is a converter arrangement that is more flexible and less costly to implement. Additionally, it would be desirable for the converter to have the capability of operating in a reverse mode for vehicles having a single machine that performs both generating and engine starting functions.
The present invention is directed to an improved motor vehicle electrical system including an engine-driven alternator and a flexible topology DC-to-DC converter coupling the alternator to the vehicle storage battery and electrical loads, where the output of the alternator is regulated based the load voltage, and the converter is operable in one of a number of different modes based on engine or alternator speed, including a forward boost mode, a forward unity mode, and a forward buck mode. In the forward boost mode, the converter output voltage is boosted above that of the alternator to enable battery charging at low engine speeds; in the forward unity mode, the alternator output voltage is transferred to the battery and electrical loads at a unity transfer ratio; and in the forward buck mode, the converter output voltage is reduced below that of the alternator to enhance the alternator power output at medium-to-high engine speeds. In applications where a single electrical machine is utilized for both starting and generating functions, the converter is capable of operation in a reverse buck mode during engine starting for purposes of increasing the current supplied to the machine.
The simple converter topology enables flexible operation at a low cost, and the control electronics is simplified because the converter switches can be modulated at a fixed frequency and duty cycle during the boost and buck modes. The availability of both forward boost and forward buck modes enables the alternator to be designed for lower voltage, higher current operation, thereby increasing its operating efficiency at higher engine speeds.